Lysosomes

Photo by: chrisharvey

Lysosomes are membrane-bound
organelles
that function as the "stomachs" of
eukaryotic cells
. They contain about fifty different
enzymes
that break down all types of biological molecules including
proteins
, nucleic acids,
lipids
, and
carbohydrates
. Cells transport material into lysosomes, the material is digested by the
enzymes, and the digested molecules are moved back into the
cytosol
for use by the cell. Both extracellular materials brought into the cell
by endocytosis and obsolete
intracellular
materials are degraded in the lysosome.

Lysosomes vary in size and shape, but have several common features. They
are surrounded by a single membrane, have an
acidic
interior pH level of about 5, and carry a high content of digestive
enzymes. All of the digestive

Three routes to degradation in lysosomes.

enzymes found in the lysosome require an acidic environment to function
properly and are called acid hydrolases. The low pH of the lysosome is
maintained by membrane proteins that pump protons (H
+
ions
) from the cytosol into the lysosome.

In addition to the proton pumps, the lysosomal membrane contains many
other proteins that transport the digested molecules out of the lysosome
and into the cytosol. Although it may seem dangerous for cells to contain
enzymes that can digest most biological molecules, the contents of the
cell are doubly protected from the digestive enzymes of the lysosome.
First, the enzymes are enclosed in the lysosomal membrane and second, even
if the enzymes were to leak out of the lysosome, they would not be active
at the neutral pH of the cytosol.

Extracellular materials to be degraded in the lysosome are brought into
the cell by either pinocytosis or phagocytosis. Pinocytosis, which occurs
in all eukaryotic cells, is the internalization of extracellular fluid and
small
macromolecules
by means of small
vesicles
that pinch off the inside of the plasma membrane. These small vesicles
carrying endocytosed molecules are initially delivered to membranous
organelles called endosomes. It is not precisely clear how molecules to be
degraded progress from endosomes to lysosomes. Endosomes may actually
mature into lysosomes when newly made acid hydrolases are delivered to the
endosome.

Phagocytosis, which occurs in only specialized cell types, is the
ingestion of large particles such as cell debris or whole microorganisms.
Phagocytic
cells engulf large particles by forming a large intracellular vesicle
containing the engulfed particle. The large vesicle then fuses with a
lysosome, resulting in a single membranous organelle in which the
digestive enzymes break down the ingested particle.

Intracellular materials, such as old organelles, are brought into a
lysosome by a process called autophagy. For example, when a mitochondrion
comes to the end of its ten-day life, it is engulfed by membrane derived
from the
endoplasmic reticulum
. The newly enclosed mitochondrion then fuses with a lysosome, resulting
in its degradation by the acid hydrolases.

A group of genetic disorders caused by defective lysosomal enzymes
demonstrates the importance of lysosomes. Called lysosomal storage
diseases, these disorders are characterized by the harmful accumulation of
undigested substances. The accumulated materials impair or kill the
affected cells, resulting in skeletal or muscular defects, mental
retardation, or even death.